Magnetism in 2- and 6-line ferrihydrite nanoparticles

Abstract

Magnetic properties of 2- and 6-line ferrihydrite (2LFh and 6LFh, respectively) as well as 2-line ferrihydrite embedded in SiO₂ (2LFh/SiO₂) in the temperature range 2–380 K are investigated using DC magnetization and AC magnetic susceptibility measurements. All the samples exhibit superparamagnetic (SPM) responses at temperatures above their respective blocking temperatures in the range T_B = 36–50 K. Linear extrapolations of 1/χ’ vs. T at 80 Hz and high temperatures give effective magnetic moment per atom μ_eff = 5.78 μ_B for 2LFh, μ_eff = 6.27 μ_B for 2LLFh/SiO₂ and μ_eff = 5.20 μ_B for 6LFh using Curie-Weiss law. The Arrhenius plots give anisotropy constants K = 4.5 × 10⁵ J/m³ for 2LFh, K = 5.5 × 10⁵ J/m³ and K = 1.5 × 10⁵ J/m³ for 6LFh using the particles size of 40, 35 and 60 Å derived from TEM data on the same samples. The number of uncompensated moments at the surfaces which contribute to the overall magnetic moments of the particles were estimated from the Néel model for antiferromagnetic nanoparticles and yielded magnetic moment per particle μ_p ∼ 283 μ_B, 251μ_B and 465 μ_B for 2LFh, 2LFh/SiO₂ and 6LFh, respectively. At 10 K, well thin the magnetic regime, the spins are blocked in specific directions resulting in usually large M_S which appears to increase with decreasing nanoparticle size. Demagnetization behaviors were used as fingerprint techniques to identify the magnetic ordering and the nature of the interactions within the samples, they confirm the existence of AFM core at temperature below T_B superimposed to a ferrimagnetic-like shell (at the surface). IRM/TRM measurements, particularly the TRM characteristic peaks at specific fields ranging from H = 10–40 kOe confirmed the existence of a spin-glass state below T_B in the samples.